The present invention relates generally to materials and processing of materials. More specifically, the present invention is directed to expandable polymers and methods for processing of expandable polymers.
A conventional method of forming an article made of an expandable polymer, such as PTFE, is to blend a powdered resin with a wettable liquid, such as a lubricant or extrusion aid, and compress the combination under relatively low pressure into a preformed billet. A wettable liquid is mixed with the powdered resin to control the degree of material shear that occurs during subsequent extrusion and to prevent excessive shear, which can damage the material.
Using a ram-type extruder, the billet is extruded through a die having a desired cross-section. Next, the wettable liquid is removed from the extruded material by drying or by another extraction method. The dried extruded material is then stretched in one or more directions at an elevated temperature below the crystalline melting point of the resin. In the case of PTFE, this results in the material taking on a microstructure characterized by elongated nodes interconnected by fibrils. Typically, the nodes are oriented with their elongated axis perpendicular to the direction of stretching.
According to conventional methods, there is a direct relationship between temperature and maximum expansion ratio while maintaining material uniformity and without breakage of the material. At low expansion temperatures, the material shows inconsistencies, is weak, and often breaks. Typically, heating well above room temperature is required to prevent the expandable polymer material from breaking and to ensure uniform material thickness after expansion.
U.S. Pat. No. 4,187,390 describes a method of forming porous PTFE that requires stretching at elevated temperatures. Material expanded at lower temperatures often fractures or results in weak material.
U.S. Pat. No. 5,552,100 describes a method of forming thin porous fluoropolymer films by post-sinter stretching the material to a final thickness less than 0.002 inches. The conventional manufacturing of films having thicknesses below 0.002 inches during pre-sinter expansion often result in breaking or tearing of the film.
Therefore, a need exists for a method providing substantial expansion of expandable polymers, without need for heating, to create uniform material with alternate polymer morphologies. Furthermore, the ability to decrease thickness, increase strength, uniformity and density of expandable polymers is desirable in many applications.
The present invention is directed generally to methods for treating expandable polymers and products produced therefrom. More particularly, the invention relates to methods for forming an article from an expandable polymer that has been stretched involving the steps of rewetting the expandable polymer with a wettable liquid to form a wetted material, and stretching the wetted material. Preferably, the wettable liquid is later removed.
According to another aspect of the invention, an article is formed by rewetting an expandable polymer and then stretching the expandable polymer.
Expandable polymer articles formed in accordance with the processes of the invention have characteristics, such as uniformity, porosity, density, node size, thickness, fibril density and permeability not attainable from conventional methods.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.